1. Technical Field
The embodiments herein generally relate to Carrier Frequency Offset (CFO) estimation in receiver system, and, more particularly to a scheme for the CFO estimation for wire-line as well as wireless communication standards in the receiver systems.
2. Description of the Related Art
Data transmission is organised as a frame in wireless standards. FIG. 1 illustrates a frame structure 100 of a preamble based standard. The frame structure 100 consists of sequence of data symbols 104 proceeded by a preamble 102. The preamble 102 is a known data and is typically used for synchronisation and channel estimation. The preamble may be modulated with M-ary Phase Shift Keying (MPSK). The MPSK modulated preambles are generated using a pseudo-random generator with constant and variable seed states resulting in constant phase preambles and variable phase preambles respectively. Typically, Mth power of M-PSK modulated preamble generates Mth harmonic carrier with no modulation. For example, S is a BPSK modulated signal and is derived as:S(n)=A(n)cos(Fc(n)+M×π)Where, M is the time that may range from 0 to n, while A(n) is time dependent amplitude and Fc(n) is a carrier frequency. Performing a squaring operation on the above equation results in the following equation:
            S      2        ⁡          (      n      )        =                              A          2                ⁡                  (          n          )                    2        [          1      +              cos        (                              2            ⁢                                          F                c                            ⁡                              (                n                )                                              +                      2            ×            m            ×            π                          ]            The above equation contains a second harmonic of the carrier with a phase modulation removed. Similarly, fourth power of a Q-PSK modulated signal will result in a fourth harmonic carrier. Carrier Frequency Offset (CFO) estimation is required for coherent demodulation of a baseband complex signal. It is also required to realise fast and reliable carrier frequency acquisition for designing high performance communication systems.
Currently, the Carrier Frequency Offset (CFO) is estimated by using a time domain based estimation followed by a Phase Lock Loop (PLL). In this scheme, the tracking bandwidth strongly depends on a loop bandwidth of the PLL. Any increase in the loop bandwidth leads to an increase in CFO estimation variance. The increased CFO estimation variance makes the CFO unreliable under a low Signal to Noise Ratio (SNR) and strong multipath condition. Additionally, the PLL based methods also suffer from slow receiver acquisition due to a large loop filter bandwidth of the PLL required for large CFO estimation. This limits the CFO acquisition range in order of KHz for a 10-20 MHz bandwidth signal.
Existing CFO estimation methods are either sensitive to a Symbol Timing Offset (STO) or require symbol time recovery prior to the CFO estimation. Such methods are not capable for concurrent estimation of CFO and Symbol Timing Offset (STO) and result in slowing down of receiver acquisition. Additionally, decision directed methods of CFO estimation use decoded samples which is achievable only if symbol timing recovery is done prior to the CFO estimation. Decision directed methods are also influenced by the low SNR since decoded symbol reliability is reduced which further reduces the estimate.
Accordingly, there remains a need for a scheme for CFO estimation which enables reliable and fast acquisition in large CFO, low SNR and strong multipath conditions.